Epitaxial structure for high-electron-mobility transistor and method for manufacturing the same

ABSTRACT

An epitaxial structure for a high-electron-mobility transistor includes a substrate, a nucleation layer, a buffer layered unit, a channel layer, and a barrier layer sequentially stacked on one another in such order. The buffer layered unit includes a plurality of p-i-n heterojunction stacks. Each of the p-i-n heterojunction stacks includes p-type, i-type, and n-type layers which are made of materials respectively represented by chemical formulas of AlxGa(1-x)N, AlyGa(1-y)N, and AlzGa(1-z)N. For each of the p-i-n heterojunction stacks, x decreases and z increases along a direction away from the nucleation layer, and y is consistent and ranges from 0 to 0.7.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of U.S. Pat.Application No. 16/947180 filed on Jul. 22, 2020, which is a bypasscontinuation-in-part (CIP) application of PCT International ApplicationNo. PCT/CN2019/073026, filed on Jan. 24, 2019, which claims priority ofChinese Invention Patent Application No. 201810071551.5, filed on Jan.25, 2018. The entire content of each of the International and Chinesepatent applications is incorporated herein by reference.

FIELD

This disclosure relates to an epitaxial structure made of semiconductormaterials, a high-electron-mobility transistor including the epitaxialstructure, and a method for manufacturing the epitaxial structure.

BACKGROUND

A gallium nitride (GaN)-based high-electron-mobility transistor (HEMT)is a heterojunction field-effect transistor including a barrier layer(i.e., a doped layer) made of aluminum gallium nitride (AlGaN) and achannel layer (i.e., an undoped layer) made of GaN. A two-dimensionalelectron gas may be formed at an interface between the barrier layer andthe channel layer (i.e., a heterojunction) since a relatively largediscontinuity of spontaneous polarization and piezoelectronicpolarization occurs at the heterojunction. Properties of the GaN-basedHEMT might largely depend on the composition (such as Al and Gacontents), thickness, and crystal quality of the barrier layer. Due toadvantages such as a relatively high concentration of thetwo-dimensional electron gas, a high rate of electron migration, and ahigh disruptive strength, the HEMT is widely used in high-pressure,high-frequency and high-temperature microwave devices, which are appliedin various fields such as aerospace, radar technology, medicine,microwave transmission, etc.

An epitaxial growth of a semi-insulating GaN-based buffer layer with ahigh quality is one of the key factors to manufacture a GaN-based HEMTdevice with satisfactory properties. It is noted that a GaN-based bufferlayer having a relatively low quality is prone to electrical leakage,which might adversely affect a pinch-off performance of the resultantHEMT device, reducing the ability of a gate-voltage to control a channelcurrent, thereby deteriorating an overall performance of the HEMTdevice. The electrical leakage of the GaN-based buffer layer might alsoincrease heat generation, which adversely affects a reliability andservice life of the HEMT device. In addition, a dislocation density ofthe GaN-based buffer layer would directly affect the rate of electronmigration of the two-dimensional electron gas in the HEMT device and aturn-on resistance thereof.

A GaN-based epitaxial structure for the HEMT device grown using metalorganic chemical vapor deposition (MOCVD) generally has a relativelyhigh background electron concentration (e.g., 10¹⁷/cm³) and exhibits arelatively low resistance. A conventional method for manufacturing aGaN-based epitaxial structure with a high resistance aims to controlepitaxial growth parameters (e.g., pressure of reaction chamber, growthtemperature, growth rate, V/III ratio, etc.) during the MOCVD process,so as to increase an amount of p-type dopant in the GaN-based epitaxialstructure and/or a concentration of defects, thereby reducing thebackground electron concentration. However, the GaN-based epitaxialstructure manufactured by such method may have an increased amount ofdefects and impurities, which greatly lowers the overall qualitythereof. In addition, the conventional method requires a high dependencyon equipments to control the epitaxial growth parameters and thus, has alow reproducibility.

Another conventional method for manufacturing the GaN-based epitaxialstructure with a high resistance is conducted by virtue of introducingmetals such as iron, chromium, and magnesium during epitaxial growth ofthe GaN-based epitaxial structure in a reaction chamber, so as togenerate a high level of defects and/or provide electron holes forreceiving excessive electrons. However, the metals might have memoryeffect, which would cause contamination of the reaction chamber, and theintroduced impurities might lower the rate of electron mobility of thetwo-dimensional electron gas in the GaN-based epitaxial structure, whichadversely affect the properties of the resultant HEMT device.

SUMMARY

Therefore, an object of the disclosure is to provide an epitaxialstructure for a high-electron-mobility transistor, ahigh-electron-mobility transistor including the epitaxial structure, anda method for manufacturing the epitaxial structure that can alleviate oreliminate at least one of the drawbacks of the prior art.

According to a first aspect of the disclosure, an epitaxial structurefor a high-electron-mobility transistor includes a substrate, anucleation layer, a buffer layered unit, a channel layer, and a barrierlayer. The nucleation layer is formed on the substrate. The bufferlayered unit is formed on the nucleation layer opposite to thesubstrate, and includes a plurality of p-i-n heterojunction stacks eachhaving a p-type layer disposed proximate to the substrate, an n-typelayer disposed distal from the substrate, and an i-type layer disposedbetween the p-type layer and the n-type layer. The p-type layer is madeof a material represented by a chemical formula of Al_(x)Ga(_(1-x))Nwhere x decreases in the p-type layer along a direction away from thenucleation layer. The i-type layer is made of a material represented bya chemical formula of Al_(y)Ga(_(1-y))N where y is consistent in thei-type layer. The n-type layer is made of a material represented by achemical formula of Al_(z)Ga(_(1-z))N where z increases in the n-typelayer along the direction away from the nucleation layer. The channellayer is formed on the buffer layered unit opposite to the nucleationlayer. The barrier layer is formed on the channel layer opposite to thebuffer layered unit.

According to a second aspect of the disclosure, a high-electron-mobilitytransistor includes a substrate, a nucleation layer, a buffer layeredunit, a channel layer, a barrier layer, a source contact, a draincontact, and a gate contact. The nucleation layer is formed on thesubstrate. The buffer layered unit is formed on the nucleation layeropposite to the substrate, and includes a plurality of p-i-nheterojunction stacks each having a p-type layer disposed proximate tothe substrate, an n-type layer disposed distal from the substrate, andan i-type layer disposed between the p-type layer and the n-type layer.The p-type layer is made of a material represented by a chemical formulaof Al_(x)Ga(_(1-x))N where x decreases in the p-type layer along adirection away from the nucleation layer. The i-type layer is made of amaterial represented by a chemical formula of Al_(y)Ga(_(1-y))N where yis consistent in the i-type layer. The n-type layer is made of amaterial represented by a chemical formula of Al_(z)Ga (_(1-z))N where zincreases in the n-type layer along the direction away from thenucleation layer. The channel layer is formed on the buffer layered unitopposite to the nucleation layer. The barrier layer is formed on thechannel layer opposite to the buffer layered unit. The source contact,the drain contact, and the gate contact are formed on the barrier layeropposite to the channel layer, and are spaced apart from one another.

According to a third aspect of the disclosure, a method formanufacturing an epitaxial structure for a high-electron-mobilitytransistor includes the steps of:

-   a) forming an nucleation layer on a substrate; and-   b) forming a buffer layered unit on the nucleation layer opposite to    the substrate, the buffer layered unit including a plurality of    p-i-n heterojunction stacks, each having a p-type layer disposed    proximate to the substrate, an n-type layer disposed distal from the    substrate, and an i-type layer disposed between the p-type layer and    the n-type layer, the p-type layer being made of a material    represented by a chemical formula of Al_(x)Ga(_(1-X))N where x    decreases in the p-type layer along a direction away from the    nucleation layer, the i-type layer being made of a material    represented by a chemical formula of Al_(y)Ga(_(1-y))N where y is    consistent in the i-type layer, the n-type layer being made of a    material represented by a chemical formula of Al_(z)Ga(_(1-z))N    where z increases in the n-type layer along the direction away from    the nucleation layer.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the disclosure will become apparent inthe following detailed description of the embodiments with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic view illustrating a high-electron-mobilitytransistor according to a first embodiment of the disclosure;

FIG. 2 is a schematic view illustrating two adjacent ones of p-i-nheterojunction stacks in the high-electron-mobility transistor accordingthe first embodiment of to the disclosure;

FIG. 3 is a schematic view illustrating a high-electron-mobilitytransistor according to a second embodiment of the disclosure;

FIG. 4 is a schematic view illustrating a high-electron-mobilitytransistor according to a third embodiment of the disclosure;

FIG. 5 is a chart illustrating an energy band gap as a function ofthickness of a buffer layered unit of the first embodiment;

FIG. 6 is a chart illustrating an energy band gap as a function ofthickness of a buffer layered unit of the second embodiment; and

FIG. 7 is a chart illustrating an energy band gap as a function ofthickness of a buffer layered unit of the third embodiment.

DETAILED DESCRIPTION

Before the disclosure is described in greater detail, it should be notedthat where considered appropriate, reference numerals have been repeatedamong the figures to indicate corresponding or analogous elements, whichmay optionally have similar characteristics.

Referring to FIGS. 1 and 2 , a high-electron mobility transistor (HEMT)according to a first embodiment of the disclosure includes an epitaxialstructure 100, a source contact 101, a drain contact 102, and a gatecontact 103. The epitaxial structure 100 includes a substrate 110, anucleation layer 120, a buffer layered unit 130, a channel layer 140,and a barrier layer 150.

The substrate 110 may be made of silicon, silicon carbide (SiC), orsapphire.

The nucleation layer 120 is formed on the substrate 110 and may be madeof gallium nitride (GaN) or aluminum nitride (AlN).

The buffer layered unit 130 is formed on the nucleation layer 120opposite to the substrate 110, and includes at least one multiplequantum well structure that contains a plurality of p-i-n heterojunctionstacks 131. Each of the p-i-n heterojunction stacks 131 includes ap-type layer 1311 that is made of a material represented by a chemicalformula of Al_(x)Ga(_(1-x))N, an i-type layer 1312 that is made of amaterial represented by a chemical formula of Al_(y)Ga(_(1-y))N, and ann-type layer 1313 that is made of a material represented by a chemicalformula of Al_(z)Ga(_(1-z))N. The p-type layers, the i-type layers, andthe n-type layers 1311, 1312, 1313 in the p-i-n heterojunction stacks131 are alternately stacked on one another along a direction away fromthe nucleation layer 120. That is to say, for each of the p-i-nheterojunction stacks 131, the p-type layer 1311 is proximate to thesubstrate 110, the n-type layer 1313 is distal from the substrate 110,and the i-type layer 1312 is disposed between the p-type layer 1311 andthe n-type layer 1313.

In some embodiments, for each of the p-i-n heterojunction stacks 131, xdecreases linearly, non-linearly, or stepwisely in the p-type layer 1311along a direction away from the nucleation layer 120; y is consistent inthe i-type layer 1312; and z increases linearly, non-linearly, orstepwisely in the n-type layer 1313 along the direction away from thenucleation layer 120.

In some embodiments, the p-type layer 1311 has a p-type proximate region13PP and a p-type distal region 13PD relative to the substrate 110; x inthe p-type proximate region 13PP ranges from 0.1 to 0.9; and x in thep-type distal region 13PD ranges from 0 to 0.7. Each of the p-typeproximate region 13PP and the p-type distal region 13PD may form aboundary with an adjacent layer or element, and a thickness of each ofthe p-type proximate region 13PP and the p-type distal region 13PD mayranges from 0 nm to a half thickness of the p-type layer 1311.

In some embodiments, y ranges from 0 to 0.7.

In some embodiments, the n-type layer 1313 has an n-type proximateregion 13NP and an n-type distal region 13ND relative to the substrate110; z in the n-type proximate region 13NP ranges from 0 to 0.7; and zin the n-type distal region 13ND ranges from 0.1 to 0.9. Each of then-type proximate region 13NP and the n-type distal region 13ND may forma boundary with an adjacent layer or element, and a thickness of each ofthe n-type proximate region 13NP and the n-type distal region 13ND mayranges from 0 nm to a half thickness of the n-type layer 1313.

In some embodiments, the p-type distal region 13PD is in contact withthe i-type layer 1312; and for each of the p-i-n heterojunction stacks131, a first absolute difference value between x in the p-type distalregion 13PD and y in the i-type layer 1312 is less than 0.1. When thefirst absolute difference value is larger than 0.1, a relatively largepolarization difference at an interface between the p-type distal region13PD and the i-type layer 1312 is less likely to generate a parasiticconduction channel therebetween.

In some embodiments, the n-type proximate region 13NP which is incontact with the i-type layer 1312; and for each of the p-i-nheterojunction stacks 131, a second absolute difference value between zin the n-type proximate region 13NP and y in the i-type layer 1312 isless than 0.1. When the second absolute difference value is larger than0.1, a relatively large polarization difference at an interface betweenthe n-type proximate region 13NP and the i-type layer 1312 is lesslikely to generate a parasitic conduction channel therebetween.

In some embodiments, for each of the p-i-n heterojunction stacks 131, athird absolute difference value between x in the p-type proximate region13PP and x in the p-type distal region 13PD is substantially equal to afourth absolute difference value between z in the n-type proximateregion 13NP and z in the n-type distal region 13ND. When the thirdabsolute difference value is substantially the same as the fourthabsolute difference value, the carriers in each of the p-i-nheterojunction stacks 131 can be well depleted when an electric field isapplied, and the growth of the layers in each of the p-i-nheterojunction stacks 131 may be controlled more easily. In addition,for each of the p-i-n heterojunction stacks 131, when the thicknesses ofthe p-type layer 1311 and the n-type layer 1313 are substantially thesame, the above effects (i.e., the depletion of the carriers and thegrowth control of the layers of the p-i-n heterojunction stacks) may befurther enhanced.

In some embodiments, as shown in FIG. 2 , a proximate one of the p-i-nheterojunction stacks 131 (which is also denoted by 131P in FIG. 2 ) anda distal one of the p-i-n heterojunction stacks 131 (which is alsodenoted by 131D in FIG. 2 ) relative to the substrate 110 (see FIG. 1 )are adjacent to each other. An average aluminum content in the proximateone of the p-i-n heterojunction stacks 131P is greater than that of thedistal one of the p-i-n heterojunction stacks 131D. In comparison to thecase that the p-in heterojunction stacks 131 each has the same averagealuminum content, by reducing the average aluminum contents of the p-i-nheterojunction stacks 131 along the direction away from the nucleationlayer 120, the stress in the buffer layered unit 130 can be wellmodulated.

In this disclosure, for each of the p-i-n heterojunction stacks 131, theaverage aluminum content refers to an average aluminum mole percent ofthe p-type layer 1311, the i-type layer 1312, and the n-type layer 1313.

In some embodiments, for each of the p-i-n heterojunction stacks 131, xgradually decreases in the p-type layer 1311 from a first value rangingbetween 0.1 and 0.9 to a second value ranging between 0 and 0.7 alongthe direction away from the nucleation layer 120; y is consistent in thei-type layer 1312 and ranges from 0 to 0.7; and z gradually increases inthe n-type layer 1313 from a third value ranging between 0 to 0.7 to afourth value ranging between 0.1 to 0.9 along the direction away fromthe nucleation layer 120.

In certain embodiments, the multiple quantum well structure includes 5to 30 of the p-i-n heterojunction stacks 131. In some embodiments, thebuffer layered unit 130 is consisted of the p-i-n heterojunction stacks131. In some embodiments, the buffer layered unit 130 includes 5 to 35of the p-in heterojunction stacks 131. For each of the p-i-nheterojunction stacks 131, the p-type layer 1311 may have a thicknessranging from 5 nm to 50 nm, the i-type layer 1312 may have a thicknessranging from 5 nm to 50 nm, and the n-type layer 1313 may have athickness ranging from 5 nm to 80 nm.

The channel layer 140 is formed on the buffer layered unit 130 oppositeto the nucleation layer 120. The channel layer is made of GaN and may beformed under conditions including a surface temperature ranging from1000° C. to 1150° C., and a pressure ranging from 100 mbar to 400 mbar.In certain embodiments, the channel layer 140 may have a thicknessranging from 50 nm to 500 nm.

The barrier layer 150 is formed on the channel layer 140 opposite to thebuffer layered unit 130. The barrier layer 150 may be made of a materialrepresented by a chemical formula of Al_(t)Ga(_(1-t))N, where 0.15 < t <0.3. The barrier layer 150 may be formed under a surface temperatureranging from 1000° C. to 1150° C. The barrier layer 150 may have athickness ranging from 10 nm to 35 nm.

The source contact 101, the drain contact 102, and the gate contact 103are formed on the barrier layer 150 opposite to the channel layer 140,and are spaced apart from one another. In some embodiments, an isolationlayer 104 is formed to fill a space among the source contact 101, thedrain contact 102, and the gate contact 103.

In some embodiments, each of the source contact 101 and the draincontact 102 may be made of nickel, titanium, titanium, alloys thereof,or other suitable materials. The gate contact 103 may be made oftitanium, platinum, tungsten, chromium, alloys thereof, silicidesthereof, or other suitable materials.

A method for manufacturing the epitaxial structure 100 of the HEMTaccording to the first embodiment of the disclosure includes thefollowing steps a) to d).

In step a), the nucleation layer 120 is formed on the substrate 110using, e.g., a metal organic chemical vapor deposition (MOCVD) process.

In certain embodiments, the nucleation layer 120 may be an AlN layerthat is grown under a relatively high surface temperature ranging from1000° C. to 1200° C., and may have a thickness ranging from 100 nm to500 nm. In other embodiments, the nucleation layer 120 may be an AlNlayer that is grown under a relatively low surface temperature rangingfrom 600° C. to 800° C., and may have a thickness ranging from 10 nm to50 nm. In still certain embodiments, the nucleation layer 120 may be aGaN layer that is grown under a relatively low surface temperatureranging from 450° C. to 550° C., and may have a thickness ranging from10 nm to 30 nm.

In step b), the buffer layered unit 130 including at least one multiplequantum well structure that contains the p-i-n heterojunction stacks 131is formed on the nucleation layer 120.

In some embodiments, each of the p-i-n heterojunction stacks 131 isformed by the following sub-steps b 1) to b 3).

In sub-step b 1), the p-type layer 1311 made of a material representedby a chemical formula of Al_(x)Ga₍₁₋ _(x))N, is formed by polarizationdoping, where x gradually decreases from a first value ranging between0.1 and 0.9 to a second value ranging between 0 and 0.7 along thedirection away from the nucleation layer 120.

In certain embodiments, the p-type layer 1311 may be formed under thefollowing conditions: a flow rate of an aluminum source (such astrimethyl aluminum (TMAl) and triethyl aluminum (TEAl)) graduallydecreasing from a level ranging between 50 sccm and 600 sccm to a levelranging between 0 sccm and 500 sccm, a flow rate of a gallium source(such as trimethyl gallium (TMGa) and triethyl gallium (TEGa)) graduallyincreasing from a level ranging between 20 sccm and 80 sccm to a levelranging between 25 sccm and 400 sccm, a flow rate of ammonia (NH₃)ranging from 1500 sccm to 20000 sccm, a surface temperature ranging from1000° C. to 1100° C., and a growth rate ranging from 0.5 µm/h to 3 µm/h.Due to the polarization effect, the p-type layer 1311 has a graduallydecreasing A1 content.

In sub-step b 2), the i-type layer 1312 made of a material representedby a chemical formula of Al_(y)Ga (₁₋ _(y))N is grown epitaxially on thep-type layer 1311, where y is consistent and ranges from 0 to 0.7.

In certain embodiments, the i-type layer 1312 is formed under thefollowing conditions: a flow rate of TMAl ranging from 0 sccm to 500sccm, a flow rate of TMGa ranging from 25 sccm to 400 sccm, a flow rateof NH₃ ranging from 1500 sccm to 30000 sccm, a surface temperatureranging from 1000° C. to 1100° C., and a growth rate ranging from 1 µm/hto 3 µm/h.

In sub-step b 3), the n-type layer 1313 made of a material representedby a chemical formula of Al_(z)Ga(₁₋ _(z))N is formed on the i-typelayer 1312 by polarization doping, where z gradually increases from athird value ranging between 0 and 0.7 to a fourth value ranging between0.1 to 0.9 along the direction away from the nucleation layer 120.

In certain embodiments, the n-type layer 1313 is formed under thefollowing conditions: a flow rate of TMAl gradually increasing from alevel ranging between 0 sccm and 500 sccm to a level ranging between 50sccm and 600 sccm, a flow rate of TMGa gradually decreasing from a levelranging between 25 sccm and 400 sccm to a level ranging between 20 sccmand 80 sccm, a flow rate of NH₃ ranging from 1500 sccm to 20000 sccm, asurface temperature ranging from 1000° C. to 1100° C., and a growth rateranging from 0.5 µm/h to 3 µm/h. Due to the polarization effect, then-type layer 1313 has a gradually increasing A1 content.

In certain embodiments, the sub-steps b 1) to b 3) may be repeated forpredetermined times (e.g., 5 to 30 times), so as to obtain the multiplequantum well including 5 to 30 of the p-i-n heterojunction stacks 131.

In step c), the channel layer 140 is formed on the buffer layered unit130. Step c) may be performed under the following conditions: a flowrate of TMGa ranging from 60 sccm to 400 sccm, a flow rate of NH₃ranging from 1500 sccm to 30000 sccm, a surface temperature ranging from950° C. to 1100° C., and a growth rate ranging from 0.5 µm/h to 3 µm/h.

In step d), the barrier layer 150 is formed on the channel layer 140.Step d) may be performed under the following conditions: a flow rate ofTMAl ranging from 80 sccm to 250 sccm, a flow rate of TMGa ranging from60 sccm to 150 sccm, a flow rate of NH₃ ranging from 6000 sccm to 12000sccm, a surface temperature ranging from 1020° C. to 1100° C., and agrowth rate ranging from 0.3 µm/h to 3 µm/h.

The detailed growth conditions for forming each layer of the firstembodiment of the epitaxial structure 100 is described below.

To be specific, in step a), a SiC substrate 110 having a chemicalmechanical planarization (CMP) polished silicon (Si) surface and anoptically polished carbon (C) surface and having a diameter of 6 inchesand a thickness of 500 µm, is first subjected to a heating treatment at1050° C. for 10 minutes so as to remove oxidants and impurities adsorbedon the stepped surface and to expose the stepped surface. Next, thenucleation layer 120 made of AlN and having a thickness of around 150 nmis formed on the exposed stepped surface of the SiC substrate 110 usingMOCVD, which is performed under the following conditions: a surfacetemperature of 1100° C., a flow rate of TMAl being 250 sccm, a flow rateof NH₃ being 3000 sccm, a pressure of 70 mbar, a growth rate of 0.3µm/h, and a growth time of 30 minutes.

In step b), the buffer layered unit 130 including one multiple quantumwell structure having an average aluminum content of 5 mol%, is formedon the nucleation layer 120 using MOCVD. The multiple quantum wellstructure contains 20 of the p-i-n heterojunction stacks 131, each ofwhich is formed as follows.

In substep b 1), the p-type layer 1311, which is made of gradedAl_(x)Ga(_(1-x))N, x gradually decreasing from 0.15 to 0.05 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed by polarization doping under the followingconditions: a flow rate of TMAl linearly decreasing from 200 sccm to 50sccm, a flow rate of TMGa linearly increasing from 150 sccm to 300 sccm,a flow rate of NH₃ increasing from 9000 sccm to 15000 sccm (i.e., thealuminum content decreasing gradually form 15 mol% to 5 mol%) , asurface temperature of 1050° C., and a growth time of 30 seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly decreases from 200 sccm to 50 sccm, and the flow rate ofTMGa stepwisely or non-linearly increases from 150 sccm to 300 sccm, thealuminum content in the p-type layer 1311 may decrease stepwisely ornon-linearly from 15 mol% to 5 mol%.

In substep b 2), the i-type layer 1312, which is made of GaN and has athickness of 30 nm, is formed on the p-type layer 1311 under thefollowing conditions: a flow rate of TMGa being 200 sccm, a flow rate ofNH₃ being 20000 sccm, a surface temperature of 1050° C., and a growthtime of 1 minute.

In substep b 3), the n-type layer 1313, which is made of gradedAl_(z)Ga(_(1-z))N, z gradually increasing from 0.05 to 0.15 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed on the i-type layer 1312 by polarization dopingunder the following conditions: a flow rate of TMAl linearly increasingfrom 50 sccm to 200 sccm, a flow rate of TMGa linearly decreasing from300 sccm to 200 sccm, a flow rate of NH₃ decreasing from 15000 sccm to9000 sccm (i.e., the aluminum content gradually increasing from 5 mol%to 15 mol%), a surface temperature of 1050° C., and a growth time of 30seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly increases from 50 sccm to 200 sccm, and the flow rate ofTMGa stepwisely or non-linearly decreases from 300 sccm to 200 sccm, thealuminum content in the n-type layer 1313 may increase stepwisely ornon-linearly from 5 mol% to 15 mol%.

The sub-steps b 1) to b 3) are repeated 20 times to obtain 20 of thep-i-n heterojunction stacks 131, and the resultant buffer layered unit130 has a total thickness of 1.2 µm and an energy band gap as shown inFIG. 5 .

Referring to FIG. 5 , “h” represents the thickness (i.e., the verticaldistance measured from the nucleation layer 120) of each of the p-i-nheterojunction stacks 131. That is, h in this embodiment is 60 nm (i.e.,the p-type layer 1311 is 15 nm, the i-type layer 1312 is 30 nm, and then-type layer 1313 is 15 nm). For each of the p-i-n heterojunction stacks131, the p-type layer 1311 has a decreasing energy band gap (i.e., aline with a negative slope) due to a gradually decreasing aluminumcontent (from 15% to 5%) thereof, the i-type layer has a consistentenergy band gap (i.e., the substantially horizontal line), and then-type layer 1313 has an increasing energy band gap (i.e., a line with apositive slope) due to a gradually increasing aluminum content (from 5%to 15%) thereof. It can be seen that, the overall energy band gap of thebuffer layered unit 130 is relatively low due to the low averagealuminum content (5%) thereof. With a low energy band gap, the bufferlayered unit 130 may serve as a back barrier layer, and may prevent aconcentration of two-dimensional electron gas to be formed from beingadversely affected.

In step c), the channel layer 140 having a thickness of 300 nm is formedon the buffer layered unit 130 under the following conditions: a flowrate of TMGa being 200 sccm, a flow rate of NH3 being 30000 sccm, asurface temperature of 1060° C., a pressure of 200 mbar, and a growthrate of 2 µm/h.

In step d), the barrier layer 150, which is made of Al_(0.25)Ga_(0.75)Nand has a thickness of 25 nm, is formed on the channel layer 140 underthe following conditions: a flow rate of TMAl being 200 sccm, a flowrate of TMGa being 90 sccm, a flow rate of NH₃ being 9000 sccm, asurface temperature of 1060° C., a pressure of 75 mbar, a growth rate of0.6 µm/h, and a growth time of 2.5 minutes.

FIG. 3 illustrates a HEMT according to a second embodiment of thedisclosure. The second embodiment is similar to the first embodiment,except that the buffer layered unit 130 in the second embodiment furtherincludes a high-resistance GaN layer 160 which is disposed on the p-i-nheterojunction stacks 131 opposite to the substrate 110, and issandwiched between the multiple quantum well structure and the channellayer 140. That is, the multiple quantum well structure and the highresistance GaN layer 160 cooperatively form the buffer layered unit 130.The high-resistance GaN layer 160 may have a thickness ranging from 500nm to 3000 nm and a resistance value greater than 10 ⁸ ohm.

A method for manufacturing the epitaxial structure 100 of the HEMTaccording to the second embodiment of the disclosure is similar to themethod for manufacturing the epitaxial structure 100 of the firstembodiment, except that the method for manufacturing the epitaxialstructure 100 of the second embodiment further includes, before step c)and after step b), a step e) of forming the high resistance GaN layer160 on the multiple quantum well structure, which may be performed underthe following conditions: a flow rate of TMGa ranging from 100 sccm to500 sccm, a flow rate of NH₃ ranging from 10000 sccm to 15000 sccm, asurface temperature ranging from 950° C. to 1000° C., a pressure rangingfrom 10 mbar to 80 mbar, and a growth rate ranging from 1.5 µm/h to 3µm/h.

The detailed growth conditions for forming each layer of the secondembodiment of the epitaxial structure 100 is described below.

To be specific, in step a), a silicon substrate 110 having a steppedsurface, a diameter of 6 inches and a thickness of 1 mm is firstsubjected to a heating treatment at 1050° C. for 15 minutes, so as toremove oxidants and impurities adsorbed on the stepped surface and toexpose the stepped surface. Next, the nucleation layer 120 made of AlNand having a thickness of around 200 nm is formed on the exposed steppedsurface of the silicon substrate 110 using MOCVD, which is performedunder similar conditions for forming the nucleation layer 120 of thefirst embodiment, except that the growth time is increased to 40minutes.

In step b), the buffer layered unit 130, which includes one multiplequantum well structure having an average aluminum content of 20% isformed on the nucleation layer 120 using MOCVD. The multiple quantumwell structure contains 15 of the p-i-n heterojunction stacks 131, eachof which is formed as follows.

In substep b 1), the p-type layer 1311, which is made of gradedAl_(x)Ga(_(1-x))N, x gradually decreasing from 0.75 to 0.05 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed by polarization doping under the followingconditions: a flow rate of TMAl linearly decreasing from 450 sccm to 50sccm, a flow rate of TMGa linearly increasing from 30 sccm to 300 sccm,a flow rate of NH₃ increasing from 1500 sccm to 10000 sccm (i.e., thealuminum content decreasing gradually from 75 mol% to 5 mol%), a surfacetemperature of 1050° C., and a growth time of 45 seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly decreases from 450 sccm to 50 sccm, and the flow rate ofTMGa stepwisely or non-linearly increases from 30 sccm to 300 sccm, thealuminum content in the p-type layer 1311 may decrease stepwisely ornon-linearly from 75 mol% to 5 mol%.

In substep b 2), the i-type layer 1312, which is made of GaN and has athickness of 30 nm, is formed on the p-type layer 1311 under the sameconditions as those for forming the i-type layer 1312 of the firstembodiment.

In substep b 3), the n-type layer 1313, which is made of gradedAl_(z)Ga(_(1-z))N, z gradually increasing from 0.05 to 0.75 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed on the i-type layer 1312 by polarization dopingunder the following conditions: a flow rate of TMAl linearly increasingfrom 50 sccm to 450 sccm, a flow rate of TMGa linearly decreasing from300 sccm to 30 sccm, a flow rate of NH₃ decreasing from 10000 sccm to1500 sccm (i.e., the aluminum content increasing gradually from 5 mol%to 75 mol%), a surface temperature of 1050° C., and a growth time of 45seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly increases from 50 sccm to 450 sccm, and the flow rate ofTMGa stepwisely or non-linearly decreases from 300 sccm to 30 sccm, thealuminum content in the n-type layer 1313 may increase stepwisely ornon-linearly from 5 mol% to 75 mol%.

The substeps b 1) to b 3) are repeated 15 times to obtain 15 of thep-i-n heterojunction stacks 131, and the resultant buffer layered unit130 has a total thickness of 0.9 µm and an energy band gap as shown inFIG. 6 .

Referring to FIG. 6 , it can be seen that the buffer layered unit 130 ofthe second embodiment has a higher energy band gap compared to that ofthe first embodiment since the buffer layered unit 130 of the secondembodiment has a higher average aluminum content (20 mol%). In addition,as compared to the first embodiment, the p-type layer 1311 of each thep-i-n heterojunction stacks 131 in the second embodiment has an energyband gap which decreases more rapidly (i.e., a line having a steepernegative slope) since the aluminum content of the p-type layer 1311decreases from 75 mol% to 5 mol% within the same thickness (i.e., 15 nm)as that of the p-type layer 1311 of the first embodiment. The n-typelayer 1313 has an energy band gap which increases more rapidly (i.e., aline having a steeper positive slope) since the aluminum content in then-type layer 1313 increases from 5 mol% to 75 mol% within the samethickness (i.e., 15 nm) as that of the n-type layer 1313 of the firstembodiment. The i-type layer 1312 has a consistent energy band gap,which is represented by a substantially horizontal line. The compressivestrain to be built due to the lattice mismatch among the nucleationlayer 120 , the buffer layered unit 130, and the channel layer 140 isused to compensate the tensile strain generated due to thermal expansionmismatch between GaN and silicon during a cooling process after theepitaxial growth under high temperature.

In step e), the high-resistance GaN layer 160, which has a thickness of2000 nm, is formed on the buffer layered unit 130 under the followingconditions: a flow rate of TMGa being 200 sccm, a flow rate of NH₃ being120000 sccm, a surface temperature of 980° C., a pressure of 50 mbar, agrowth rate of 2.5 µm/h, and a growth time of 50 minutes.

In step c), the channel layer 140, which has a thickness of 200 nm, isformed on the high-resistance GaN layer 160 under similar conditions asthose for forming the channel layer 140 of the first embodiment.

In step d), the barrier layer 150, which is made of Al_(0.25)Ga_(0.75)Nand has a thickness of 25 nm, is formed on the channel layer 140 undersimilar conditions as those for forming the barrier layer 150 of thefirst embodiment.

FIG. 4 illustrates a HEMT according to a third embodiment of thedisclosure. The third embodiment is similar to the second embodiment,except that the buffer layered unit 130 of the third embodiment includesa plurality of multiple quantum well structures. In some embodiments,the buffer layered unit 130 includes first to third multiple quantumwell structures 130 a, 130 b, 130 c. The first multiple quantum wellstructure 130 a includes at least one pi-n heterojunction stack 131A,the second multiple quantum well structure 130 b includes at least onepi-n heterojunction stack 131B, and the third multiple quantum wellstructure 130 c includes at least one pi-n heterojunction stack 131C. Inother words, the p-i-n heterojunction stacks 131 of the buffer layeredunit 130 of the third embodiment (including the pi-n heterojunctionstacks 131A, 131B, 131C) are arranged in a plurality of sets (which arealso denoted by 130 a, 130 b, 130 c).

In some embodiments, at least one of the sets 130 a, 130 b, 130 cincludes at least two of the p-i-n heterojunction stacks 131A, 131B,131C each of which has an average aluminum content. The average aluminumcontents in the at least two of the p-i-n heterojunction stacks 131A,131B, 131C are the same with each other.

In addition, each of the multiple quantum well structures (i.e., each ofthe sets 130 a, 130 b, 130 c) has an average aluminum content. The sets130 a, 130 b, which are adjacent to each other, are proximate to anddistal from the substrate 110, respectively. The sets 130 b, 130 c,which are adjacent to each other, are proximate to and distal from thesubstrate 110, respectively. When a proximate one of the sets 130 a, 130b, 130 c and a distal one of the sets 130 a, 130 b, 130 c relative tothe substrate 110 are adjacent to each other, an average aluminumcontent in the proximate one of the sets 130 a, 130 b, 130 c is greaterthan that of the distal one of the sets 130 a, 130 b, 130 c. In otherwords, the aluminum contents in the multiple quantum well structuresdecrease in a stepwise manner along the direction away from thenucleation layer 120. In comparison to the case that the multiplequantum well structures each has the same average aluminum content, byreducing the average aluminum contents of the multiple quantum wellstructures along the direction away from the nucleation layer 120, thestress in the buffer layered unit 130 can be well modulated.

In some embodiments, the p-i-n heterojunction stacks 131 are arranged inthe sets, for example the first set 130 a, the second set 130 b, and thethird set 130 c. Each of the sets includes at least one of the p-i-nheterojunction stacks 131. At least one of the sets includes at leasttwo of the p-i-n heterojunction stacks 131, each of which has an averagealuminum content. The average aluminum contents in the at least two ofthe p-i-n heterojunction stacks 131 are the same with each other. When aproximate one of the sets and a distal one of the sets relative to thesubstrate 110 are adjacent to each other, an average aluminum content inthe proximate one of the sets is greater than that of the distal one ofthe sets.

The detailed growth conditions for forming each layer in the epitaxialstructure 100 of the third embodiment is described below.

To be specific, in step a), the nucleation layer 120 is formed on thesilicon substrate 110 under the same conditions as those for forming thenucleation layer 120 of the second embodiment.

In step b), the buffer layered unit 130, which includes the firstmultiple quantum well structure 130 a having an average aluminum contentof 72.5 mol%, the second multiple quantum well structure 130 b having anaverage aluminum content of 47.5 mol%, and the third multiple quantumwell structure 130 c having an average aluminum content of 22.5 mol%, isformed on the nucleation layer 120 using MOCVD.

To be specific, the first multiple quantum well structure 130 a isfirstly formed on the nucleation layer 120, and contains 15 of the p-i-nheterojunction stacks 131, each of which is formed as follows.

In substep b 1), the p-type layer 1311, which is made of gradedAl_(x)Ga(_(1-x))N, x gradually decreasing from 0.9 to 0.7 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed by polarization doping under the followingconditions: a flow rate of TMAl linearly decreasing from 450 sccm to 350sccm, a flow rate of TMGa linearly increasing from 20 sccm to 30 sccm, aflow rate of NH₃ increasing from 1500 sccm to 2000 sccm (i.e., thealuminum content decreasing gradually from 90 mol% to 70 mol%), asurface temperature of 1050° C., and a growth time of 80 seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly decreases from 450 sccm to 350 sccm, and the flow rate ofTMGa stepwisely or non-linearly increases from 20 sccm to 30 sccm, thealuminum content in the p-type layer 1311 may decrease stepwisely ornon-linearly from 90 mol% to 70 mol%.

In substep b 2), the i-type layer 1312, which is made ofAl_(0.65)Ga_(0.35) N (i.e., Al_(y)Ga(_(1-y))N, where y = 0.65) and has athickness of 30 nm, is formed on the p-type layer 1311 under thefollowing conditions: a flow rate of TMAl being 350 sccm, a flow rate ofTMGa being 38 sccm, and a flow rate of NH₃ being 1500 sccm, a surfacetemperature of 1050° C., and a growth time of 3 minutes.

In substep b 3), the n-type layer 1313, which is made of gradedAl_(z)Ga(_(1-z)) N, z gradually increasing from 0.7 to 0.9 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed on the i-type layer 1312 by polarization dopingunder the following conditions: a flow rate of TMAl linearly increasingfrom 350 sccm to 450 sccm, a flow rate of TMGa linearly decreasing from30 sccm to 20 sccm, a flow rate of NH₃ decreasing from 2000 sccm to 1500sccm (i.e., the aluminum content increasing gradually from 70 mol% to 90mol%), a surface temperature of 1050° C., and a growth time of 80seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly increases from 350 sccm to 450 sccm, and the flow rate ofTMGa stepwisely or non-linearly decreases from 30 sccm to 20 sccm, thealuminum content in the n-type layer 1313 may increase stepwisely ornon-linearly from 70 mol% to 90 mol%.

The substeps b 1) to b 3) are repeated 3 times to obtain 3 of the p-i-nheterojunction stacks 131, and the resultant first multiple quantum wellstructure 130 a has a total thickness of 180 nm (i.e., “3h₁” shown inFIG. 7 ).

Next, the second multiple quantum well structure 130 b is formed on thefirst multiple quantum well structure 130 a, and contains 8 of the p-i-nheterojunction stacks 131B, each of which is formed as follows.

In substep b 1), the p-type layer 1321, which is made of gradedAl_(x)Ga(_(1-x))N, x gradually decreasing from 0.65 to 0.45 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed by polarization doping under the followingconditions: a flow rate of TMAl linearly decreasing from 350 sccm to 300sccm, a flow rate of TMGa linearly increasing from 38 sccm to 60 sccm, aflow rate of NH₃ increasing from 1500 sccm to 1800 sccm (i.e., thealuminum content decreasing gradually from 65 mol% to 45 mol%), asurface temperature of 1050° C., and a growth time of 75 seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly decreases from 350 sccm to 300 sccm, and the flow rate ofTMGa stepwisely or non-linearly increases from 38 sccm to 60 sccm, thealuminum content in the p-type layer 1321 may decrease stepwisely ornon-linearly from 65 mol% to 45 mol%.

In substep b 2), the i-type layer 1322, which is made ofAl_(0.4)Ga_(0.6) N (i.e., Al_(y)Ga(_(1-y))N, where y = 0.4) and has athickness of 30 nm, is formed on the p-type layer 1321 under thefollowing conditions: a flow rate of TMAl being 300 sccm, a flow rate ofTMGa being 74 sccm, a flow rate of NH₃ being 3000 sccm, a surfacetemperature of 1050° C., and a growth time of 2 minutes.

In substep b 3), the n-type layer 1323, which is made of gradedAl_(z)Ga(_(1-z)) N, z gradually increasing from 0.45 to 0.65 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed on the i-type layer 1322 by polarization dopingunder the following conditions: a flow rate of TMAl linearly increasingfrom 300 sccm to 350 sccm, a flow rate of TMGa linearly decreasing from60 sccm to 38 sccm, a flow rate of NH₃ decreasing from 1800 sccm to 1500sccm (i.e., the aluminum content increasing gradually from 45 mol% to 65mol%), a surface temperature of 1050° C., and a growth time of 75seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly increases from 300 sccm to 350 sccm, and the flow rate ofTMGa stepwisely or non-linearly decreases from 60 sccm to 38 sccm, thealuminum content in the n-type layer 1323 may increase stepwisely ornon-linearly from 45 mol% to 65 mol%.

The substeps b 1) to b 3) are repeated 8 times to obtain 8 of the p-i-nheterojunction stacks 131B, and the resultant second multiple quantumwell structure 130 b has a total thickness of 480 nm (i.e., “8h₂” shownin FIG. 7 ) .

Then, the third multiple quantum well structure 130 c is formed on thesecond multiple quantum well structure 130 b, and contains 15 of thep-i-n heterojunction stacks 131C, each of which is formed as follows.

In substep b 1), the p-type layer 1331, which is made of gradedAl_(x)Ga(_(1-x))N, x gradually decreasing from 0.4 to 0.2 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed by polarization doping under the followingconditions: a flow rate of TMAl linearly decreasing from 300 sccm to 250sccm, a flow rate of TMGa linearly increasing from 74 sccm to 130 sccm,a flow rate of NH₃ increasing from 1500 sccm to 2000 sccm (i.e., thealuminum content gradually decreasing from 40 mol% to 20 mol%), asurface temperature of 1050° C., and a growth time of 55 seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly decreases from 300 sccm to 250 sccm, and the flow rate ofTMGa stepwisely or non-linearly increases from 74 sccm to 130 sccm, thealuminum content in the p-type layer 1331 may decrease stepwisely ornon-linearly from 40 mol% to 20 mol%.

In step b 2), the i-type layer 1332, which is made ofAl_(0.15)Ga_(0.85)N (i.e., Al_(y)Ga(_(1-y))N, where y = 0.15) and has athickness of 30 nm, is formed on the p-type layer 1331 under thefollowing conditions: a flow rate of TMAl being 350 sccm, a flow rate ofTMGa being 240 sccm, a flow rate of NH₃ being 6000 sccm, a surfacetemperature of 1050° C., and a growth time of 1 minute.

In substep b 3), the n-type layer 1333, which is made of gradedAl_(z)Ga(_(1-z)) N, z gradually increasing from 0.2 to 0.4 along thedirection away from the nucleation layer 120, and which has a thicknessof 15 nm, is formed on the i-type layer 1332 by polarization dopingunder the following conditions: a flow rate of TMAl linearly increasingfrom 250 sccm to 300 sccm, a flow rate of TMGa linearly decreasing from130 sccm to 74 sccm, a flow rate of NH₃ decreasing from 2000 sccm to1500 sccm (i.e., the aluminum content increasing gradually from 20 mol%to 40 mol%), a surface temperature of 1050° C., and a growth time of 55seconds.

In some other embodiments, when the flow rate of TMAl stepwisely ornon-linearly increases from 250 sccm to 300 sccm, and the flow rate ofTMGa stepwisely or non-linearly decreases from 130 sccm to 74 sccm, thealuminum content in the n-type layer 1333 may increase stepwisely ornon-linearly from 20 mol% to 40 mol%.

The substeps b 1) to b 3) are repeated 15 times to obtain 15 of thep-i-n heterojunction stacks 133, and the resultant third multiplequantum well structure 130 c has a total thickness of 0.9 µm (i.e.,“15h₃” shown in FIG. 7 ).

Referring to FIG. 7 , it can be seen that an energy band gap of thebuffer layered unit 130 of the third embodiment decreases with anincreased thickness thereof (i.e., in the direction away from thenucleation layer 120). As the average aluminum contents in the multiplequantum well structures 130 a, 130 b, 130 c decrease, the energy bandgap of the buffer layered unit 130 decreases correspondingly. By formingthe buffer layered unit 130 that includes a plurality of multiplequantum well structures having graded average aluminum contents, thetensile strain generated due to thermal expansion mismatch between GaNand silicon during a cooling process after the epitaxial growth underhigh temperature may be effectively compensated by the multiple quantumwell structures 130 a, 130 b, 130 c of the buffer layered unit 130 dueto the compressive strain to be built therein.

It should be noted that, the number of the multiple quantum well in thebuffer layered unit 130 and the graded aluminum content thereof are notlimited to those disclosed above, and may be modified according topractical requirements.

In steps e), c), and d), the high-resistance GaN layer 160, the channellayer 140, and the barrier layer 150 of the third embodiment are formedunder the same conditions as those for forming the high-resistance GaNlayer 160, the channel layer 140, and the barrier layer 150 of thesecond embodiment.

In sum, the epitaxial structure for the high-electron-mobilitytransistor and the manufacturing method thereof according to the presentdisclosure have the following advantages.

First of all, since there is a great difference in spontaneouspolarization coefficients between AlN-based material and GaN-basedmaterial (0.029 C/m² to 0.081 C/m²), epitaxial growth of graded AlGaNmaterials with gradually decreasing (or increasing) A1 and Ga contentscan generate negative (or positive) residual polarization charges due tothe difference in polarized intensities, so as to achieve n/p typedoping (i.e., polarization doping) without introducing donor/acceptordopants, thereby obtaining p-type AlGaN layer (or n-type AlGaN layer) .Therefore, by forming the buffer layered unit 130 using polarizationdoping, the problem of contamination of the reaction chamber due tomemory effect of the impurities generated by metallic dopants present inthe conventional method for forming the GaN-based buffer layer, may beavoided. In addition, the process for manufacturing the epitaxialstructure using polarization doping can be better controlled since it isa relatively simple process compared to the conventional method formanufacturing the conventional epitaxial structure.

Secondly, due to a built-in electric field generated in the p-i-nheterojunction stacks 131A that includes polarization doped n-type andp-type graded AlGaN layers 1311, 1313 and i-type AlGaN layers 1312 withfixed composition, a background carrier concentration contained thereinmay be greatly reduced, thereby forming the buffer layered unit 130which has a relatively high resistance and high quality, and which isless prone to electrical leakage.

Lastly, by forming the buffer layered unit 130 that includes a pluralityof multiple quantum well structures having graded average aluminumcontents, the compressive strain generated by the lattice mismatch amongthe buffer layered unit 130, the channel layer 140 (GaN), and thenucleation layer (AlN) 120 can be used to effectively buffer the tensilestrain generated due to thermal expansion mismatch between the channellayer 140 (GaN) and the substrate (Si) during a cooling process afterepitaxial growth under high temperature.

In the description above, for the purposes of explanation, numerousspecific details have been set forth in order to provide a thoroughunderstanding of the embodiments. It will be apparent, however, to oneskilled in the art, that one or more other embodiments may be practicedwithout some of these specific details. It should also be appreciatedthat reference throughout this specification to “one embodiment,” “anembodiment,” an embodiment with an indication of an ordinal number andso forth means that a particular feature, structure, or characteristicmay be included in the practice of the disclosure. It should be furtherappreciated that in the description, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure and aiding in theunderstanding of various inventive aspects, and that one or morefeatures or specific details from one embodiment may be practicedtogether with one or more features or specific details from anotherembodiment, where appropriate, in the practice of the disclosure.

While the disclosure has been described in connection with what areconsidered the exemplary embodiments, it is understood that thisdisclosure is not limited to the disclosed embodiments but is intendedto cover various arrangements included within the spirit and scope ofthe broadest interpretation so as to encompass all such modificationsand equivalent arrangements.

What is claimed is:
 1. An epitaxial structure for ahigh-electron-mobility transistor, comprising: a substrate; a nucleationlayer formed on said substrate; a buffer layered unit which is formed onsaid nucleation layer opposite to said substrate, and which includes aplurality of p-i-n heterojunction stacks each having a p-type layerdisposed proximate to said substrate, an n-type layer disposed distalfrom said substrate, and an i-type layer disposed between said p-typelayer and said n-type layer, said p-type layer being made of a materialrepresented by a chemical formula of Al_(x)Ga_((1-x))N where x decreasesin said p-type layer along a direction away from said nucleation layer,said i-type layer being made of a material represented by a chemicalformula of Al_(y)Ga₍₁₋ _(y))N where y is consistent in said i-typelayer, said n-type layer being made of a material represented by achemical formula of Al_(z)Ga_((1-z)) N where z increases in said n-typelayer along the direction away from said nucleation layer; a channellayer formed on said buffer layered unit opposite to said nucleationlayer; and a barrier layer formed on said channel layer opposite to saidbuffer layered unit.
 2. The epitaxial structure according to claim 1,wherein said buffer layered unit is consisted of said p-i-nheterojunction stacks.
 3. The epitaxial structure according to claim 1,wherein said p-type layer has a p-type proximate region and a p-typedistal region relative to said substrate, x in said p-type proximateregion ranging from 0.1 to 0.9, x in said p-type distal region rangingfrom 0 to 0.7.
 4. The epitaxial structure according to claim 1, whereiny ranges from 0 to 0.7.
 5. The epitaxial structure according to claim 1,wherein said n-type layer has an n-type proximate region and an n-typedistal region relative to said substrate, z in said n-type proximateregion ranging from 0 to 0.7, z in said n-type distal region rangingfrom 0.1 to 0.9.
 6. The epitaxial structure according to claim 1,wherein said p-type layer has a p-type distal region which is in contactwith said i-type layer, and for each of said p-i-n heterojunctionstacks, an absolute difference value between x in said p-type distalregion and y in said i-type layer is less than 0.1.
 7. The epitaxialstructure according to claim 1, wherein said n-type layer has an n-typeproximate region which is in contact with said i-type layer, and foreach of said p-i-n heterojunction stacks, an absolute difference valuebetween z in said n-type proximate region and y in said i-type layer isless than 0.1.
 8. The epitaxial structure according to claim 1, whereinsaid p-type layer has a p-type proximate region and a p-type distalregion relative to said substrate, said n-type layer has an n-typeproximate region and an n-type distal region relative to said substrate,and for each of said p-i-n heterojunction stacks, an absolute differencevalue between x in said p-type proximate region and x in said p-typedistal region is equal to an absolute difference value between z in saidn-type proximate region and z in said n-type distal region.
 9. Theepitaxial structure according to claim 1, wherein said buffer layeredunit includes 5 to 35 of said p-i-n heterojunction stacks.
 10. Theepitaxial structure according to claim 1, wherein when a proximate oneof said p-i-n heterojunction stacks and a distal one of said pi-nheterojunction stacks relative to said substrate are adjacent to eachother, an average aluminum content in said proximate one of said pi-nheterojunction stacks is greater than that of said distal one of saidp-i-n heterojunction stacks.
 11. The epitaxial structure according toclaim 1, wherein said p-i-n heterojunction stacks are arranged in aplurality of sets, each of said sets including at least one of saidp-i-n heterojunction stacks, at least one of said sets including atleast two of said p-i-n heterojunction stacks, each of which has anaverage aluminum content, the average aluminum contents in said at leasttwo of said p-i-n heterojunction stacks being the same with each other,and when a proximate one of said sets and a distal one of said setsrelative to said substrate are adjacent to each other, an averagealuminum content in said proximate one of said sets is greater than thatof said distal one of said sets.
 12. The epitaxial structure accordingto claim 1, wherein said buffer layered unit further includes ahigh-resistance GaN layer which is disposed on said p-i-n heterojunctionstacks opposite to said substrate, and which has a resistance valuegreater than 10⁸ ohm.
 13. A high-electron-mobility transistor,comprising: a substrate; a nucleation layer formed on said substrate; abuffer layered unit which is formed on said nucleation layer opposite tosaid substrate, and which includes a plurality of p-i-n heterojunctionstacks each having a p-type layer disposed proximate to said substrate,an n-type layer disposed distal from said substrate, and an i-type layerdisposed between said p-type layer and said n-type layer, said p-typelayer being made of a material represented by a chemical formula ofAl_(x)Ga₍₁ _(-x))N where x decreases in said p-type layer along adirection away from said nucleation layer, said i-type layer being madeof a material represented by a chemical formula of Al_(y)Ga₍₁₋ _(y))Nwhere y is consistent in said i-type layer, said n-type layer being madeof a material represented by a chemical formula of Al_(z)Ga₍ _(1-z)) Nwhere z increases in said n-type layer along the direction away fromsaid nucleation layer; a channel layer formed on said buffer layeredunit opposite to said nucleation layer; a barrier layer formed on saidchannel layer opposite to said buffer layered unit; and a sourcecontact, a drain contact, and a gate contact which are formed on saidbarrier layer opposite to said channel layer, and which are spaced apartfrom one another.
 14. The high-electron-mobility transistor according toclaim 13, wherein said buffer layered unit is consisted of said p-i-nheterojunction stacks.
 15. The high-electron-mobility transistoraccording to claim 13, wherein said p-type layer has a p-type distalregion which is in contact with said i-type layer, and for each of saidp-i-n heterojunction stacks, an absolute difference value between x insaid p-type distal region and y in said i-type layer is less than 0.1.16. The high-electron-mobility transistor according to claim 13, whereinsaid n-type layer has an n-type proximate region which is in contactwith said i-type layer, and for each of said p-i-n heterojunctionstacks, an absolute difference value between z in said n-type proximateregion and y in said i-type layer is less than 0.1.
 17. Thehigh-electron-mobility transistor according to claim 13, wherein saidp-type layer has a p-type proximate region and a p-type distal regionrelative to said substrate, said n-type layer has an n-type proximateregion and an n-type distal region relative to said substrate, and foreach of said p-i-n heterojunction stacks, an absolute difference valuebetween x in said p-type proximate region and x in said p-type distalregion is equal to an absolute difference value between z in said n-typeproximate region and z in said n-type distal region.
 18. Thehigh-electron-mobility transistor according to claim 13, wherein when aproximate one of said p-i-n heterojunction stacks and a distal one ofsaid p-i-n heterojunction stacks relative to said substrate are adjacentto each other, an average aluminum content in said proximate one of saidpi-n heterojunction stacks is greater than that of said distal one ofsaid p-i-n heterojunction stacks.
 19. The high-electron-mobilitytransistor according to claim 13, wherein said p-i-n heterojunctionstacks are arranged in a plurality of sets, each of said sets includingat least one of said p-i-n heterojunction stacks, at least one of saidsets including at least two of said p-i-n heterojunction stacks, each ofwhich has an average aluminum content, the average aluminum contents insaid at least two of said p-i-n heterojunction stacks being the samewith each other, and when a proximate one of said sets and a distal oneof said sets relative to said substrate are adjacent to each other, anaverage aluminum content in said proximate one of said sets is greaterthan that of said distal one of said sets.
 20. A method formanufacturing an epitaxial structure for a high-electron-mobilitytransistor, comprising the steps of: a) forming an nucleation layer on asubstrate; and b) forming a buffer layered unit on the nucleation layeropposite to the substrate, the buffer layered unit including a pluralityof p-i-n heterojunction stacks, each having a p-type layer disposedproximate to the substrate, an n-type layer disposed distal from thesubstrate, and an i-type layer disposed between the p-type layer and then-type layer, the p-type layer being made of a material represented by achemical formula of Al_(x)Ga_((1-x))N where x decreases in the p-typelayer along a direction away from the nucleation layer, the i-type layerbeing made of a material represented by a chemical formula ofAl_(y)Ga₍₁₋ _(y))N where y is consistent in the i-type layer, the n-typelayer being made of a material represented by a chemical formula ofAl_(z)Ga_((1-z))N where z increases in the n-type layer along thedirection away from the nucleation layer.